Rotational redistribution effect on saturated laser-induced fluorescence

A number of papers have recently appeared which discuss the use of saturated laser-induced fluorescence (LIF) as a means of measuring local densities of atoms and molecules, based mostly on a two-level model. In this paper, a simplified four-level model is developed which incorporates most of the physics reported by Lucht and Laurendeau (1979) but which allows closed-form solutions for the emission intensity in terms of just two rotational parameters for each electronic state. It shows that rotational redistribution can have a significant effect on both the concentration and the quenching constants deduced by saturated LIF, in agreement with Lucht and Laurendeau. The four-level model is described after a brief review of the two-level model, so that a comparison can be made between the predictions of the two models.